cording to their periods and amplitudes; but these, instead of being available in any particular form, are always more or less complex. If we could produce waves of just the rate and amplitude we desire, without any others in combination, a great step would be gained. Then we could produce light without wasting at the same time a great amount of energy in producing heat which we do not want. This is one of the subordinate problems awaiting solution. If to the production of such waves as are wanted we could add a means of recording and fixing them in their true relative proportion, we would have the solution of another great and fascinating subordinate problem—the exact reproduction of natural scenes in color. A long step has been taken toward accomplishing the first of these achievements in the remarkable experiments by Mr. Tesla with alternating electrical currents of high frequency and high potential. Among the startling facts brought out in these experiments is that although a current of electricity, either direct or alternating, from ordinary dynamos under fifteen hundred or two thousand volts electro-motive 'force will kill, yet under alternations of a million to a million and a half per second a voltage of fifty thousand produces no shock or injury. Electric lamps light with but a single wire leading to them. Vacuum tubes become luminous in a properly prepared room with no wires, and it is not extravagant, in view of what has already appeared, to predict a future when unlimited power will be available at every man's hand. That will be when, as Mr. Tesla says, we are able to "hook our machinery to the machinery of Nature." In the conclusion of his lecture before the Institution of Electrical Engineers, London, after describing a plan by which he thinks it would be practicable to telephone across the Atlantic, he adds: "But such cables will not be constructed, for, ere long, intelligence—transmitted without wires—will throb through the earth like a pulse through a living organism. The wonder is that, with the present state of knowledge and the experience gained, no attempt is being made to disturb the electrostatic or magnetic condition of the earth, and transmit, if nothing else, intelligence." It is probable that this wonder will give place to a still greater at no distant period, by reason of successful attempts of just the kind here mentioned. The problem is already in course of solution, the distinguished electrician, Mr. Preece, having recently succeeded in sending telephonic messages over a circuit which was wholly disconnected from that in which the generator was placed, and at a distance of three miles from it.
Unquestionably one of the most powerful aids to investigation of late has been photography. Both as a science and as an art it has grown in precision, speed, and availability, until now it has become a weapon of attack as well as a means of record. While